Pro has been shown to play an important role in ameliorating environmental stress in plants and microorganisms, including heavy metal stress. Here, we describe the effects of the expression of a mothbean ¹-pyrroline-5-carboxylate synthetase (P5CS) gene in the green microalga Chlamydomonas reinhardtii . We show that transgenic algae expressing the mothbean P5CS gene have 80% higher free-Pro levels than wild-type cells, grow more rapidly in toxic Cd concentrations (100 µM), and bind fourfold more Cd than wild-type cells. In addition, Cd-K edge extended x-ray absorption fine structure studies indicated that Cd does not bind to free Pro in transgenic algae with increased Pro levels but is coordinated tetrahedrally by sulfur of phytochelatin. In contrast to P5CS-expressing cells, Cd is coordinated tetrahedrally by two oxygen and two sulfur atoms in wild-type cells. Measurements of reduced/oxidized GSH ratios and analyses of levels of malondialdehyde, a product of the free radical damage of lipids, indicate that free Pro levels are correlated with the GSH redox state and malondialdehyde levels in heavy metal-treated algae. These results suggest that the free Pro likely acts as an antioxidant in Cd-stressed cells. The resulting increased GSH levels facilitate increased phytochelatin synthesis and sequestration of Cd, because GSH-heavy metal adducts are the substrates for phytochelatin synthase.